coreboot: add timer queue implementation

A timer queue provides the mechanism for calling functions
in the future by way of a callback. It utilizes the MONOTONIC_TIMER
to track time through the boot. The implementation is a min-heap
for keeping track of the next-to-expire callback.

Change-Id: Ia56bab8444cd6177b051752342f53b53d5f6afc1
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3158
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/Kconfig

@@ -318,6 +318,12 @@ config HAVE_MONOTONIC_TIMER
 	help
 	 The board/chipset provides a monotonic timer.
 
+config TIMER_QUEUE
+	def_bool n
+	depends on HAVE_MONOTONIC_TIMER
+	help
+	  Provide a timer queue for performing time-based callbacks. 
+
 config HIGH_SCRATCH_MEMORY_SIZE
 	hex
 	default 0x0

src/include/timer.h

@@ -38,6 +38,17 @@ struct rela_time {
 	long microseconds;
 };
 
+/* A timeout_callback structure is used for the book keeping for scheduling
+ * work in the future. When a callback is called the structure can be
+ * re-used for scheduling as it is not being tracked by the core timer
+ * library any more. */
+struct timeout_callback {
+	void *priv;
+	void (*callback)(struct timeout_callback *tocb);
+	/* Not for public use. The timer library uses the fields below. */
+	struct mono_time expiration;
+};
+
 /* Obtain the current monotonic time. The assumption is that the time counts
  * up from the value 0 with value 0 being the point when the timer was
  * initialized.  Additionally, the timer is assumed to only be valid for the
@@ -49,6 +60,13 @@ struct rela_time {
  * of 10 seconds. */
 void timer_monotonic_get(struct mono_time *mt);
 
+/* Returns 1 if callbacks still present in the queue. 0 if no timers left. */
+int timers_run(void);
+
+/* Schedule a callback to be ran microseconds from time of invocation.
+ * 0 returned on success, < 0 on error. */
+int timer_sched_callback(struct timeout_callback *tocb, unsigned long us);
+
 /* Add microseconds to an absoute time. */
 static inline void mono_time_add_usecs(struct mono_time *mt, long us)
 {

src/lib/Makefile.inc

@@ -90,6 +90,7 @@ ramstage-$(CONFIG_COLLECT_TIMESTAMPS) += timestamp.c
 ramstage-$(CONFIG_COVERAGE) += libgcov.c
 ramstage-$(CONFIG_MAINBOARD_DO_NATIVE_VGA_INIT) += edid.c
 ramstage-y += memrange.c
+ramstage-$(CONFIG_TIMER_QUEUE) += timer_queue.c
 
 # The CBMEM implementations are chosen based on CONFIG_DYNAMIC_CBMEM.
 ifeq ($(CONFIG_DYNAMIC_CBMEM),y)

src/lib/timer_queue.c

@@ -0,0 +1,198 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright (C) 2013 Google, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+#include <stddef.h>
+#include <timer.h>
+
+#define MAX_TIMER_QUEUE_ENTRIES 64
+
+/* The timer queue is implemented using a min heap. Therefore the first
+ * element is the one with smallest time to expiration. */
+struct timer_queue {
+	int num_entries;
+	int max_entries;
+	struct timeout_callback *queue[MAX_TIMER_QUEUE_ENTRIES];
+};
+
+static struct timer_queue global_timer_queue = {
+	.num_entries = 0,
+	.max_entries = MAX_TIMER_QUEUE_ENTRIES,
+	.queue = { 0 },
+};
+
+static inline int timer_queue_empty(struct timer_queue *tq)
+{
+	return tq->num_entries == 0;
+}
+
+static inline int timer_queue_full(struct timer_queue *tq)
+{
+	return tq->num_entries == tq->max_entries;
+}
+
+static inline struct timeout_callback *timer_queue_head(struct timer_queue *tq)
+{
+	if (timer_queue_empty(tq))
+		return NULL;
+	return tq->queue[0];
+}
+
+static int timer_queue_insert(struct timer_queue *tq,
+                              struct timeout_callback *tocb)
+{
+	int index;
+
+	/* No more slots. */
+	if (timer_queue_full(tq))
+		return -1;
+
+	index = tq->num_entries;
+	tq->num_entries++;
+	tq->queue[index] = tocb;
+
+	while (index != 0) {
+		struct timeout_callback *parent;
+		int parent_index;
+
+		parent_index = (index - 1) / 2;
+		parent = tq->queue[parent_index];
+
+		/* All other ancestors are less than or equal to the current. */
+		if (mono_time_cmp(&parent->expiration, &tocb->expiration) <= 0)
+			break;
+
+		/* The parent is greater than current. Swap them. */
+		tq->queue[parent_index] = tocb;
+		tq->queue[index] = parent;
+
+		index = parent_index;
+	}
+
+	return 0;
+}
+
+/* Get the index containing the entry with smallest value. */
+static int timer_queue_min_child_index(struct timer_queue *tq, int index)
+{
+	int left_child_index;
+	int right_child_index;
+
+	left_child_index = 2 * index + 1;
+
+	if (left_child_index >= tq->num_entries)
+		return -1;
+
+	right_child_index = left_child_index + 1;
+
+	if (right_child_index >= tq->num_entries)
+		return left_child_index;
+
+	if (mono_time_cmp(&tq->queue[left_child_index]->expiration,
+	                 &tq->queue[right_child_index]->expiration) < 0) {
+		return left_child_index;
+	}
+	return right_child_index;
+}
+
+static void timer_queue_remove_head(struct timer_queue *tq)
+{
+	int index;
+	struct timeout_callback *tocb;
+
+	/* In order to remove the head the deepest child is replaced in the
+	 * head slot and bubbled down the tree. */
+	tq->num_entries--;
+	tocb = tq->queue[tq->num_entries];
+	tq->queue[0] = tocb;
+
+	index = 0;
+	while (1) {
+		int min_child_index;
+		struct timeout_callback *child;
+
+		min_child_index = timer_queue_min_child_index(tq, index);
+
+		/* No more entries to compare against. */
+		if (min_child_index < 0)
+			break;
+
+		child = tq->queue[min_child_index];
+
+		/* Current index is the correct place since it is smaller or
+		 * equal to the smallest child. */
+		if (mono_time_cmp(&tocb->expiration, &child->expiration) <= 0)
+			break;
+
+		/* Need to swap with smallest child. */
+		tq->queue[min_child_index] = tocb;
+		tq->queue[index] = child;
+
+		index = min_child_index;
+	}
+}
+
+static struct timeout_callback *
+timer_queue_expired(struct timer_queue *tq, struct mono_time *current_time)
+{
+	struct timeout_callback *tocb;
+
+	tocb = timer_queue_head(tq);
+
+	if (tocb == NULL)
+		return NULL;
+
+	/* The timeout callback hasn't expired yet. */
+	if (mono_time_before(current_time, &tocb->expiration))
+		return NULL;
+
+	timer_queue_remove_head(tq);
+
+	return tocb;
+}
+
+int timer_sched_callback(struct timeout_callback *tocb, unsigned long us)
+{
+	struct mono_time current_time;
+
+	if ((long)us< 0)
+		return -1;
+
+	timer_monotonic_get(&current_time);
+	tocb->expiration = current_time;
+	mono_time_add_usecs(&tocb->expiration, us);
+
+	/* The expiration overflowed. */
+	if (us != 0 && !mono_time_before(&current_time, &tocb->expiration))
+		return -1;
+
+	return timer_queue_insert(&global_timer_queue, tocb);
+}
+
+int timers_run(void)
+{
+	struct timeout_callback *tocb;
+	struct mono_time current_time;
+
+	timer_monotonic_get(&current_time);
+	tocb = timer_queue_expired(&global_timer_queue, &current_time);
+
+	if (tocb != NULL)
+		tocb->callback(tocb);
+
+	return !timer_queue_empty(&global_timer_queue);
+}